Obduction

Obduction was originally defined by Coleman to mean the overthrusting of oceanic lithosphere onto continental lithosphere at a convergent plate boundary where continental lithosphere is being subducted beneath oceanic lithosphere.

Subsequently, this definition has been broadened to mean the emplacement of continental lithosphere by oceanic lithosphere at a convergent plate boundary, such as closing of an ocean or a mountain building episode. This process is uncommon because the denser oceanic lithosphere usually subducts underneath the less dense continental plate. Obduction occurs where a fragment of continental crust is caught in a subduction zone with resulting overthrusting of oceanic mafic and ultramafic rocks from the mantle onto the continental crust. Obduction may occur where a small tectonic plate is caught between two larger plates, with the lithosphere (both island arc and oceanic) welding onto an adjacent continent as a new terrane. When two continental plates collide, obduction of the oceanic lithosphere between them is often a part of the resulting orogeny.

Most obductions appear to have initiated at back-arc basins above the subduction zones during the closing of an ocean or an orogeny.

The characteristic rocks of obducted oceanic lithosphere are the ophiolites. Ophiolites are an assemblage of oceanic lithosphere rocks that have been emplaced onto a continent. This assemblage consists of deep-marine sedimentary rock (chert, limestone, clastic sediments), volcanic rocks (pillow lavas, glass, ash, sheeted dykes and gabbros) and peridotite (mantle rock).

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